DR6 is a member of a death receptor family, and is also called a TNF receptor super family member 21 (TNFRSF21), or TR9. DR6 is a type-I transmembrane receptor having four extracellular cysteine rich motifs and one cytoplasmic death domain structure (Non Patent Literature 1).
Expression of DR6 is observed in tissues such as heart, brain, placenta, pancreas, lymph node, thymus, prostatic tissue, skeletalmuscle, kidney and didymus, and plays an important role in various biological control mechanisms, and also relationships with diverse diseases have been indicated.
For example, the possibility that DR6 is an aggravating factor of multiple sclerosis has been indicated (Non Patent Literature 2). Also, the possibility that DR6 is an aggravating factor of asthma has been indicated (Non Patent Literature 3).
Also there has been reported that DR6 has a function of binding with amyloid precursor protein (APP) which is believed to cause Alzheimer's disease, and controlling cell death and defective axon elongation of nerve cells (Non Patent Literature 4), and there is a patent application directed to a DR6 antagonistic antibody which is to be a DR6 antagonist that inhibits or blocks the interaction between DR6 and APP (Patent Literature 1).
Meanwhile, it is known that DR6 is deeply involved in the control of the immune mechanism. For example, the above Non Patent Literature 1 reports that overexpression of DR6 in a specific cell strain into which a gene encoding DR6 is transduced causes the activation of both NF-kB and JNK, and apoptosis.
There has been also reported that in DR6 gene-deficient model mice, T cells do not substantially have a normal function in JNK activation, and when protein antigens are administered to DR6 (−/−) mice, T cells excessively proliferate, and show excessively strong deviation to Th2 response (Non Patent Literature 5).
Further, when a model of experimental autoimmune encephalomyelitis induced by myelin oligodendrocyte glycoprotein was used, DR6 (−/−) mice showed higher tolerance to both of the onset and the progression of CNS disease in comparison with wild-type littermates. That is, there is a possibility that DR6 is involved in the infiltration of leukocyte, and has a function of inducing and progressing experimental autoimmune encephalomyelitis (Non Patent Literature 6).
Thus, DR6 is believed to be an important receptor molecule which is a starting point of intracellular signal transduction concerning immune mechanism, particularly activation regulation of peripheral T-cell. Therefore, a molecule capable of modulating the receptor function concerning immune regulation of DR6 by specifically binding with DR6 is expected to become a trigger molecule that activates or suppresses immune function via signal transduction of DR6, and can be used as an immune regulating agent.
However, DR6 does not show binding affinity to known TNF ligand family molecules, and the true nature of DR6 ligand capable of inducing a signal concerning immune regulation by specifically binding to DR6 is still unknown.
Meanwhile, Sdc1, which is also called CD138 antigen, is a transmembrane heparan sulfate proteoglycan macromolecule (HSPG) that forms a syndecan family having homology with a transmembrane domain and a cytoplasmic domain, together with syndecan 2 which is also called fibroglycan, syndecan 3 which is also called N-syndecan, and syndecan 4 which is also called amphiglycan.
HSPG shows distribution specific for cell types, and is known to be involved in interaction with extracellular matrix proteins, cell surface molecules or soluble proteins such as cytokines. In the transmembrane domain and the intracellular domain, there exist four tyrosine residues of which positions are highly conserved, and the possibility of involvement in intracellular signal transduction by phosphorylation of any one of the tyrosine residues has been indicated.
A gene encoding human Sdc1 (hSdc1) has been already cloned (Non Patent Literature 7). hSdc1 consists of 310 amino acid residues, and includes an extracellular domain of amino acids at positions 1 to 251 followed by a hydrophobic transmembrane region of 25 amino acid residues (amino acids at positions 252 to 276), and a cytoplasmic domain of the last 34 amino acid residues (amino acids at positions 277 to 310). The amino acids at positions 1 to 17 form a secretion signal, which is understood as being useful when Sdc1 is secreted from a cell that biosynthesizes hSdc1, but not being necessary for physiological activity of hSdc1.
Meanwhile, the extracellular domain of hSdc1 includes five glycosaminoglycan (GAG) adherable sites at position 37, position 45, position 47, position 206 and position 216. Among these, the amino acid residues at position 45 and position 47 are known to lose the binding ability to heparan sulfate, and to simultaneously lose the physiological activity as hSdc1 when they are substituted with other amino acid residues.
However, it is unknown that specific binding between Sdc1 and DR6 is involved in suppression of T cell activation, namely Sdc1 is a physiological ligand of DR6 regarding regulation of immune response by DR6.